Piezo-electric crystals



Jan. 8, 1929.

C. F. BRUSH, JR PIEZO ELECTRIC CRYSTALS Original Filed May 6, 1927j'nve'ntov mawyk 61cm n e 1 0 Patented Jan. 8, 1929.

PATENT OFFICE.

CHARLES F. BRUSH, JR., OF CLEVELAND HEIGHTS, OHIO; THE CLEVELAND TRUSTCOMPANY, EXECUTOR OF SAID BRUSH, 33., DECEASED, ASSIGNOR, BY MESNEASSIGNMENTS, TO THE CLEVELAND TRUST COMPANY, OF CLEVELAND, OHIO, A

CORPORATION OF OHIO.

v IPIEZO-ELECTRIC CRYSTALS.

Application filed Kay 6, 1927, Serial No. 189,440. Renewed June 15,1928.

This invention relates to piezo-electric crystals and more particularlyto a method of improving the characteristics of piezo-electric crystals,such as Rochelle salt crystals of the com 'osite type. i

The term composite crystal as applied to piezo-electric crystals,particularly Rochelle salt crystals, relates to the type of crystalhaving opaque or porous end regions. The term hour-glass is alsofrequently applied to this structure due to the shape of the opaqueregions. These regions are, in fact, of irregular crystalline structurecontaining occluded mother liquor and are the result of rapid growth ofthe crystal and may be produced, for example, by allowing thetemperature of a saturated aqueous solution of Rochelle salt to fallcomparativel rapidl solution from which suc crysta are deposited beallowed to cool more slowly, this hour-glass structure is less markedand may be eliminated entirely by extremely slow cooling. Thesehour-glass regions contain mother liquor even after extensivedesiccation of the crystal. Thus, after soaking the crystal in absolutealcohol for several days and then baking for a week at 4.0 degrees C.these hour-glass regions are still moist as may be demonstrated bcutting open the crystal. More violent esiccation, such as heating invacuo, will dry up these regions to some extent. It has long beenobserved, however, that these hour-glass regions were necessary for thesuccessful operation of this type of crystal.

In electrical work, as in sound reproducing mechanism, a pair ofterminals is usually secured to each such crystal. One such terminalusually or frequently comprises a pair of connected electrodes pressedagainst the porous or hour-glass ends of the crystal, the other terminalbeing formed by a metal foil band substantial] encircling the body ofthe crystal, as is wel known in the art. The crystal is then frequentlycoated with an insulating material, such as a varnish. The. crystal isthen .frequently mounted in a cradle, one end of the crystal beingpressed against or otherwise fixedly attached to the cradle, the otherend of the crystal being attached'to a lever arm pivoted with respect tothe cradle. With such mounting, as is well If the sary for thesuccessful operation of devices of this type.

Certain serious disadvantages accompany the use of such crystals. Thus,for example,

the relatively highly electrically conductive porous or hour-glassregions tend to dry up by spontaneous evaporation, with accompanyingdecrease in electrical conductivity and consequent decrease inpiezo-electrical actlvity. A further difliculty has been caused byleakage of the liquid from the hour-glass region, particularly after thecrystal has been subjected to violent electrical or mechanical stressesfor relatively long periods. Such leakage liquid has frequently creptaround the outside of the crystal beneath an insulating coating, such asvarnish, whereby to short-circuit the apparatus. Moreover, the porous orhour-glass regions are at best somewhat irregular and non-uniform inextent and character. Such irregularities result in non-uniformelectrical performance and sometimes result in electrical puncture dueto the breaking down of the crystalline structure intermediate an hourglass portion and an electrode, by the difference of electric potentialbetween said electrode and the porous or hour-glass region.

An object of my invention is to provide a method of treatingpiezo-electric crystals to provide a crystal structure of a high degreeof uniformity and efiiciency.

Another object of this invention is to provide a method of treatingpiezoelectric crystals to form crystal structures of uniform andpermanent character.

A further object of my invention is to provide a method whereby theporous or hourglass regions of a composite Rochelle salt crystal may bepermanently rendered highly electrically conductive.

Another object of the invention is to provide a piezo-electric crystalhaving con uctive portions of permanent and uniform character.

vide a piezo-electric crystal of increased efiic iency.

- Other objects of the invention will be apparent from the descriptionof it hereinafter, given, and the accompanying drawmgs, wherein Figure 1is a top plan view, and Fig. 2 is a perspective View, of a compositeRochelle saltcrystal before being treated according to my invention;

Fig. 3 is a bottom plan view of a composite Rochelle salt crystal,showlng apertures through the base thereof;

Fig. 4 is a sectional View illustrating one form of apparatus in whichmy invention may be carried out; a r

Fig. 5 is a bottom plan view of a comp-oslte Rochelle salt crystal afterbeing treated according to my invention; metal foil being shownassociated with the crystal;

Fig. 6 is a horizontal sectional view on line 6-6 of Fig. 7, showing acrystal from which material has been removed, one of the cavities beingfilled with conductive material, the other cavity being lined withconductive material; v

Fig. 7 is a vertical section on, line 77 of Fig. 6; y

Fig. 8 is a plan View of an acoustic device embodying a crystal treatedaccording to my invention; and Y Y Fig. 9 is a front elevation of thestructure shown in Fig. 8.

My invention may be said to comprise generally the increase ofpermanency-and efficiency of crystals of the type described by renderingthe hour-glass regions of such crystals permanently and uniformlyelectrically conductive. It will be understood, of course, that thebeneficial results of my invention may be obtained in various ways.

One manner in which my invention may be carried out contemplatesremoving the liquid material from the hour-glass regions 1 of a crystal2 and substituting therefor conducting material which can neither dry upnor leak out. Thus, for example, referring particularly, to Figs. 3 andi, there is shown a Rochelle salt crystal 2 of the composite type inwhich apertures 3 have been drilled in the base of the crystal so as tocommunicate with the hour-glass. regions 1. In Fig. 4 the crystal 2 isshown as disposed on a suitable gasket 4; disposed within an airtightchamber 5 provided with a compressed air inlet 6 and an outlet 7 Thecrystal 2 is preferably so disposed that the apertures 3 are insubstantial register with the outlet aperture 7, which outletcommunicates with the atmosphere. By introducing compressed air-throughthe inlet pipe 6 into the chamher 5, the liquid in the hour-glassregions 1 is forced out of the crystal 2 through the apertures 3. Theremoval of the liquid from the regions 1 may frequently be facilitatedby heating the crystal to a suitable temperature, such as, for example,about 40 degrees C.

It may be noted that the removal of the liquid from the hour-glassregions 1 renders such portions substantial]. electricallynonconductive, and according y a crystal which has been so treated isrelatively inactive piezo-electrically.

The hour-glass portions 1 may be rendered permanently and uniformlyelectrically conductive by disposing therein suitable conductivematerial. Such material may be introduced into the portions 1 byremoving the crystal 2 from the chamber 5, disposing the material to beintroduced adjacent the outer ends of the portions 1, and applyingvacuum to the apertures 3. The material to be introduced is accordinglysucked into the portions 1 and will fill substantially all the spacepreviously occupied by the liquid, such as mother liquor, previouslyremoved.

A suitable material for the above-described purpose is an aqueoussolution of silver nitrate, which solution, if desired, may contamammonia. After the introduction of such silver nitrate solution into.the portions 1 the crystalmay be exposed to light whereby metallicsilver will be precipitated within the portions 1. Any remainingsolution may then be removed in the manner described for the removal ofthe original liquid contained It will be further understood that theremay be introduced into the portions 1 a liquid which is in itselfconductive but which has the property of drying and becomingnon-conductive electrically when exposed to the air. Thus, for example,certain varnishes may be introduced into the portions 1 with verysatisfactory results. Thus, if material such as a varnish should leakout from the portions 1, such material would dry and become anon-conductor electrically, at the same time sealing the portions 1 andpreventing evaporation and solidification of the material containedtherein. 7

If anelectrode, such as the tin foil electrode 9, isdisposed about acrystal treated as hereinbefore mentioned, it will be understood thatsuch electrode should be cut away as indicated at 10 in Fig. 5 to avoidcontact with the conductive interior of the crystal 2. The. slightreduction in electrode area caused by the apertures 10, however, has nonoticeable effect on the piezo-electric' performance of the crystal.

My invention may be carried out by mechanically removing all or aportion of the hour-glass portions 1 and mechanically introducingtherein conducting material to form inner electrodes. Thus, in Figs. 6'and 7 the hour-glass portions are shown as mechanical- 1y removed toform the cavities 11 and 12.

Thecavity 11 is shown as substantially entirely filled with suitableconducting material 13 such as tin foil, amalgam, powdered graphite, orthe like. The cavity 12 is shown as lined with a suitable material suchas tin foil 14 to form a conductive inner electrode. In any givencrystal any or all of the cavities maybe provided with electrodes of thetype shown at 13 or at 14.

This method of carrying out my invention is advantageous in that theextent of the original hour-glass portionsl is of no great importance,since the cavities formed by mechanical removal need not coincide withtheoriginal hour-glass portions, but may be of greater or lesser extent.If of lesser extent,

the hour-glass material remaining may be rendered non-conductive byheating the crystal 2 in vacuo, thereby eliminating any danger ofelectrical puncture as indicated above. If the cavity formed by removingthe portions- 1 is greater in extent than the original por tions 1, thepiezo-electric activity of the crystal is increased due to the greaterextent of the conductive material. By carrying outthe invention in thismanner, therefore, it is possible to make the electrodes 13 or 14 of thesize, shape and character most suitable for the purpose for which thecrystal is to be used, regardless of the original extent of thehourglass regions 1.

In Figs. 8 and 9 I have shown an acoustic device embodying a crystaltreated according to my invention. Thus, the crystal 2 is shown asdisposed in cooperative relation with. the cradle 15. At each end of thecrystal are shown plastic electrically conductive portions 16 and 17disposed adjacent the electrically conductive end regions 18 and 19 ofthe crystal 2. The portion 17 is disposed in closely engaging positionwith the arm 20 of the cradle 15 whereas the portion 16 is secured tohave provided a method of treating composite crystals of the typedescribed whereby to produce piezo-electriccrystals of the desiredelectrical characteristics, and of permanent character and greatlyincreased efficiency.

It will moreover be noted that I have provided a method of producingcomposite crystals of the type described having predetermined electricalcharacteristics.

It will also be seen that by my invention composite piezo-electriccrystals of the type described may be formed of uniform and, permanentcharacter.

It will also be noted that by my invention the electrical conductivityofthe hour-glass portions of piezo-electric crystals of the type describedmay be greatly increased and rendered permanent in character.

To those skilled in the art many modifications of and widel differingembodiments and applications 0 my invention will suggest themselveswithout departing from the spirit and scope thereof. My disclosures andthe descriptions herein are purely illustrative and are not intended tobe in any sense limiting.

What I claim is:

1. The method of providingapiezo-electric crystal of improvedcharacteristics which comprises removing material from the ends of saidcrystal and associating with said ends electrically conductive materialof substantially permanent character.

2. The method of improving the characteristics of a composite crystal ofthe type described having a region containing occluded liquid, whichcomprises removing material from said region, and substituting thereforother electrically conductive material.

3. The method of improving the character istics of a composite crystalof the type described having a ,region containing liquid, whichcomprises removing material from said region, and substituting thereforsolid electrically conductive material. 1

4. The method of improving the characteristics of a composite crystal ofthe type described having a region containing liquid, which comprisesremoving said liquid from said region and substituting thereforelectrically conductive material.

5. The method of improving the characteristics of acomposite crystal ofthe type described having a region containing liquid,

which-comprises removing the liquid substantially completely from saidregion, removing a portion ofsaid region, and substituting for saidremoved portion material of substantially constant electricalconductivity.

6. Themethod of improving the characteristics of a composite crystal ofthe type described having a region containing liquid,

which comprises removing liquid from said region, introducing into saidregion a solution capable of yielding an-electrically conductiveprecipitate, precipitating an electrically conductive material from saidsolution, and removing the excess solution whereby the precipitatedconductive material Wlll remain in said region.

7. The method of improving the characteristics of a composite crystal ofthe type de-- scribed having a region containing liquid, which comprisesremoving said liquid substantially completely from said region, intro-CHARLES F. BRUSH, JR.

